In the Frieza Saga of DBZ, when the Ginyu Force attacks Goku, he's sooo fast that it looks like he's absorbing all the attacks, but actually moving so quickly that it's barely noticeable, and we're shown his footprints which tell that he's moving at mind-boggling velocity.

In the battle with Frieza as well, they move so fast that it's barely noticeable to Krillin who's a human being. Considering that Goku's powers have risen exponentially since then and when the Androids arrive on earth, the fighting sequences which depict them just do not have the velocity it had in Frieza Saga. The only way we're left to perceive that Androids are stronger than Frieza is because the story tells us that and because a small blow the androids give makes a huge blow for Goku/Vegeta (before they enter hyperbolic time chamber).

Also, if just turning super saiyan and a few blows from the transformed frieza destroyed the planet namek - then how does planet earth survive for so long with cell and super saiyan 2 levels? How much stronger is earth than namek?

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So, especially with Dragonball Z and beyond, a certain suspension of plot has to be done.

Because everyone is always becoming more and more powerful, it would be just boring to show a still of the fighters but then explain it with "oh well they're moving so fast it just looks like they're standing still".

It would be boring to watch.

As the series progresses they find more and more ways to show just how impactful and powerful the fights and fighters are. With foreknowledge of how strong the last foe was, to see them just knock the hero away is an adequate way of showing that they mean business.

Also, as we go on in the timeline all the fighers become more and more powerful, but they're always on some kind of relative playing ground. For instance, by the time Majin Buu comes into play, all the core Z fighters have a base power level about even with Frieza.

Basically as the series goes on, its just something to expect, they don't keep to the full cannon as perfectly as they should, but they have to show the power some way.

As far as why Namek was destroyed, Frieza specifically fired a special beam attack into Namek's core in order to disrupt it and destroy the planet. None of the Z fighters nor their enemies has done something like this, so while the Earth is getting plenty beat up, it's not being blown up.

Until Kid Buu surfaces during the tail end of the Buu saga and destroys it with one giant blast that is. But that is also a blast of such magnitude to demonstrate just how powerful this new foe is.